When any resource of a network device involved in the processing of data packets receives packets at a higher rate than it can process the packets, then packets will back up, effectively giving rise to a queue of data packets. Flow control mechanisms are generally provided to manage the transmission into the queue of data packets from various flows, where a “flow” is some defined set or category of data packets, such as packets from a particular group of sources, or conforming to a particular protocol, or packets requiring a particular class of service for example. In particular, queue management schemes can be implemented to control the flow rate into the queue (i.e. the enqueuing rate or “serviced rate”) for each flow according to the availability of bandwidth in the resource. Bandwidth availability is typically assessed by some form of queue length thresholding technique which indicates whether excess bandwidth (i.e. spare or unused bandwidth) is available in operation. Flow rates are then controlled accordingly, with a view to ensuring fair sharing of bandwidth between flows while meeting any guaranteed service specifications for particular flows. For example, Active Queue Management (AQM) schemes such as RED (Random Early Detection) and variations thereof control flow rates by probabilistically dropping input packets (or marking packets to indicate network congestion), the drop probabilities for respective flows being increased or decreased in a defined way based on queue levels. A more recent AQM scheme known as Bandwidth Allocation Technology (BAT) is described in “Bandwidth Allocation for Non-Responsive Flows with Active Queue Management”, Bowen et al., Int. Zurich Seminar on Broadband Communications, IZS 2002, February 2002. This scheme belongs to the family of additive-increase multiplicative-decrease (AIMD) control algorithms, and monitors excess bandwidth by sensing not only queue levels but also rates of queue-level changes.
Queue management schemes thus control flow rates based on local congestion levels, adjusting flow rates in dependence on excess bandwidth in the particular local resource to which the queue corresponds. Additional flow control mechanisms may also operate on a network-wide level. In particular, certain network protocols are classed as “responsive”. Responsive protocols provide some mechanism for adjusting flow rates based on events, such as dropping or marking of packets, indicative of congestion status occurring elsewhere in the network. For example, Internet protocols such as TCP (Transport Control Protocol) incorporate an end-to-end back-off mechanism whereby devices send data at lower rates when congestion is sensed. Depending on the particular mechanism employed, different responsive protocols can provide different levels of responsiveness, but all provide for some degree of congestion control at wider-than-local level. Other network protocols such as UDP (User Datagram Protocol) are classed as non-responsive. These protocols continue sending data at the same rate regardless of congestion in the network. Non-responsive protocols thus rely on local mechanisms to handle congestion control.
A typical network may support traffic conforming to a variety of different protocols, and may thus carry both responsive and non-responsive traffic. In general, therefore, it is desirable for network flow control mechanisms to deal efficiently and in a fair manner with a mix of different traffic types.